1. Field of the Invention
The present invention relates generally to fluid filters with a filter unit of a porous ceramic medium, and particularly to those useful in minute or micron fluid filtration, ultra fluid filtration and reverse-osmosis fluid separation, and gas separation.
2. Description of the Background Art
Conventionally, in processes for manufacturing foods, chemicals, semiconductors and the like, filter units (also referred to simply as xe2x80x9cunitsxe2x80x9d hereinafter) formed of porous organic membranes are used to filter fluids. An organic membrane, however, is less resistant to heat, pressure and chemicals and it is thus being replaced with a membrane formed of a porous ceramic medium which is highly resistant to heat, pressure and chemicals. Furthermore, in recent years such ceramic membrane has also been applied to a bio-reactor such as a carrier for microorganism incubation, a catalyst carrier, and the like.
Recently there has been an increased demand particularly for such a porous ceramic membrane that has a significantly high permeability and also has a high resistance for stress capable of resisting a pressure of a fluid even if the membrane has a reduced thickness. In order for a membrane to have an enhanced permeability the membrane must have its porosity increased to reduce its resistance given to a fluid permeating therethrough and also have its pores reduced in diameter to allow finer components in the fluid to be separated. In other words, the membrane requires enhanced separation capability. Normal ceramic conventionally used, such as alumina (Al2O3), can, however, only provide for a porous medium having a porosity of at most 40%, normally at most 30% or therearound. This results in more of pressure drop through the membrane when it is filtered. Thus such a unit as only making use of its pores is inefficient and thus does not meet the demand as described above.
To overcome such disadvantage, a porous ceramic medium can be molded in the form of a cylinder having a multi-layered structure to allow a fluid to in practice permeate through a membrane of a reduced thickness and thus minimize its pressure drop. More specifically, a collection of units, or a filter module (also referred to as a xe2x80x9cmodulexe2x80x9d hereinafter), formed of the porous ceramic medium, has a multi-layered structure configured of a porous membrane with filtration capability, a base supporting the membrane, and an intermediate, posed between the membrane and the base if required.
To put such modules to practical use, a unit must be designed to have a structure to allow a fluid permeating through a membrane to have an increased flow per unit permeation-area of the membrane and it also must have its external dimensions minimized. To achieve this goal, a monolith structure has been developed. Such unit has a cross section, as shown in FIG. 23. In the figure, a filter unit 1 is in the form of a lotus root, with a porous ceramic medium 2 having provided therein a large number of channels or cells 3 passing a feed fluid. Note that a xe2x80x9cfeed fluidxe2x80x9d refers to a fluid before it is filtered and a xe2x80x9cpermeated fluidxe2x80x9d refers to a fluid after it is filtered. Exemplary monolith structures are disclosed in Japanese Patent National Publication Nos. 1-501534 and 3-500386 and Swiss Patent CH 681281. A monolith structure also basically has a multi-layered structure having a porous membrane and a base supporting the membrane. Providing a multi-layered structure internal to a channel of a module having such a cross section as shown in FIG. 23, however, requires a rather complicated process and hence an additional cost. Instead, if a porous medium of the above-mentioned normal ceramic is provided having a mono-layer structure to allow a fluid permeating through a membrane with small pressure drop, the membrane must have a reduced thickness. In practice it is no more than 1 mm, although such thickness significantly reduces the membrane""s resistance for stress.
WO 94/27929 (Japanese Patent National Republication No. 06-827929) discloses a porous medium of silicon nitride based ceramic (a ceramic containing silicon nitride and/or sialon as a main component(s)) normally having a high porosity of no less than 50% as well as high resistance for stress. This porous ceramic medium is formed of silicon nitride and/or sialon grains each in the form of a column having an aspect ratio of no less than three, and of a binder of oxide, and the column grains of silicon nitride and/or sialon are bound together directly or via the binder of oxide to form a three-dimensional, randomly expanding network structure having a 3-point bending strength of at least 100 MPa. Such porous ceramic medium ensures that a thin membrane of a mono-layer structure, such as that described above, can have high resistance for stress. It should be noted, however, that simply replacing a monolith unit with a unit of silicon nitride based ceramic having a mono-layer structure, does not result in the unit having a filtered flow increased as expected, since as shown in FIG. 23 by a curved line a fluid must move a long distance particularly from a channel located around the center of the unit to the outer periphery of the unit and the fluid thus results in more of its pressure drop, so that the unit has its total permeated flow determined mainly by the permeated flow only from the channels adjacent to the surface of the unit.
In processes for manufacturing foods, chemicals and the like, such monolith module is normally used in cross-flow filtration in various manners. In this filtration system, as shown in FIG. 24, a feed fluid 5 in a source-fluid vessel 4 is fed by means of a feed pump 6 to a filter provided with a module 1, and a permeated fluid is discharged and thus collected through an outlet of the casing while a feed fluid that has passed through a channel or cell internal to the module is returned via a circulatory line 7 to the source-fluid vessel and re-fed to the filter repeatedly and the fluid is thus filtered.
The present inventors have also proposed in Japanese Patent Laying-Open No. 11-123308 a module having a small resistance against a permeating fluid and thus suitable for the above filtration system, formed of a thin membrane having a mono-layer structure of the above silicon nitride based ceramic medium.
For a filter employing such monolith filter module, it is important to increase its desired permeated flow (a flow rate of a permeated fluid per unit time, i.e., filtration rate). In order to do so, a collection of units, or a module, is required to pass a fluid with less of the fluid""s pressure drop. However, the module is also required to have a membrane having high separation capability depending on its pores"" diameter, as has been described above. Thus the two requirements must be well-balanced. Furthermore, since normally a large number of units are arranged and housed in a casing of a fixed volume their volumes cannot be arbitrarily increased to provide an enhanced permeated flow.
In order to meet the demands as above, it is essential to, with units arranged in a casing, (1) increase the amount of a fluid permeated per unit surface area of a membrane, (2) reduce the fluid""s pressure drop caused at a discharging route, and to (3) increase the membrane""s surface area effective in filtration. Exemplary modules with collected and arranged units are disclosed, e.g., in Japanese Patent Publication No. 6-11370 and Japanese Patent Laying-Open No. 5-146609, although they mainly address issue (3) only and also have each module providing for a limited permeated flow. On the other hand, the module proposed in Japanese Patent Laying-Open No. 11-123308 has its individual units overcoming issues (1) and (2), although it fails to satisfactorily address issue (3).
In order to overcome the above three disadvantages simultaneously, the present inventors, while making use of the module structure proposed in Japanese Patent Laying-open No. 11-123308, have tried to achieve a reasonable arrangement of units in the module particularly to achieve an enhanced permeated flow, and as a result the present inventors have arrived at the present invention.
To achieve the above goal, in accordance with the present invention a filter includes a casing having a cross section passing a feed fluid, and a plurality of filter units housed in the casing, formed of a porous ceramic medium and having a cross section passing the feed fluid. The filter unit has a plurality of channels defined by a wall surface of the porous ceramic medium and extending in a predetermined direction, a discharge port formed at an outer peripheral wall surface of the porous ceramic medium, and a discharge hole. The channel includes an open channel penetrating in the predetermined direction and a closed channel having opposite ends sealed as seen in the predetermined direction. The discharge hole is not provided at the wall defining the open channel but the wall defining the closed channel. The closed channel communicates with another closed channel through the discharge hole to reach the discharge port. The filter units have a cross section corresponding to at least 35% of an internal cross section of the casing. In the present invention preferably the filter units have a cross section corresponding to at least 60% of the internal cross section of the casing.
The above configuration ensures that a membrane has a sufficient surface area per unit volume of the casing to provide an increased permeated flow.
In the present invention still preferably the casing has a generally circular cross section and the discharge port includes a first discharge port formed at one outer peripheral wall surface of the porous ceramic medium and a second discharge port formed at the other outer peripheral wall surface of the porous ceramic medium and communicating with the first discharge port through the discharge hole. In the plurality of filter units between the first and second discharge ports there is a distance average of no more than half an inner diameter of a cross section of the casing. In the present invention, still preferably, in the plurality of filter units between the first and second discharge ports there is a distance average of no less than one tenth and no more than one fourth of the inner diameter of the cross section of the casing.
With such configuration, a fluid that passes and permeates through the discharge hole can minimize its pressure drop to provide for an increased permeated flow.
In the present invention, preferably the open channel and the closed channel are each linearly aligned and arranged as seen in a cross section of the filter unit. In the present invention, still preferably a row or column of the open channel linearly aligned and a row or column of the closed channel linearly aligned are alternately arranged. In the present invention, still preferably the closed channel communicates with another closed channel linearly through the discharge hole to reach the discharge port.
With such configuration, a feed fluid that flows into an open channel partially flows through a solid portion of the porous ceramic medium and enters a closed channel aligned in a row or column adjacent to the open channel. As such a fluid once filtered can flow freely in the closed channel until it is discharged through the discharge hole, and the fluid thus does not receive a resistance when it permeates. Furthermore the filter units occupying more than 35% in cross section of the casing ensure that a membrane has a sufficient surface area per unit volume of the casing to provide an increased permeated flow.
In the present invention, preferably the wall is allowed to have a thickness varying to xc2x120% around its thickness average. With such configuration, a fluid flowing therethrough can have a pressure drop balanced in a cross section of the casing to provide a uniform and hence increased permeated flow.
In the present invention, preferably a plurality of the filter units varying in the size of cross section are combined and arranged in the casing. With such configuration, a filter membrane can be increased in surface area per unit volume of the volume occupied by the entirety of the filter, to provide an increased permeated flow.
In the present invention, preferably the porous ceramic medium is formed of silicon nitride based ceramic which is formed of silicon nitride grains and/or sialon grains each in the form of a column having an aspect-ratio average of at least three, and of a binder of oxide, and which has a porosity from 30 to 70%, an permeation-diameter average from 0.01 to 10 xcexcm, and a 3-point bending strength of at least 100 MPa. With such configuration, a grain smaller than a maximal pore diameter observable according to mercury porosimetry can also be filtered, since in contrast to ceramic other than silicon nitride based ceramic, such as alumina-based ceramic, having pores of a round cross section in general, silicon nitride based ceramic has pores of an elongate cross section in the form of a slit. As such, alumina-based ceramic cannot filter a grain smaller than the maximal pore diameter observable according to mercury porosimetry, whereas silicon nitride based ceramic can filter a grain, for example, of approximately one fifth of the maximal pore diameter observable according to mercury porosimetry. Furthermore the silicon nitride based ceramic with the above characteristics can also provide for a pore diameter average of approximately 0.04 xcexcm, observable according to mercury porosimetry.
In the present invention, preferably the casing has opposite ends provided with an inlet and an outlet for passing a feed fluid, and an externally connectable connector, and has a side provided with an externally connectable opening and a discharge port discharging a permeated fluid. With such configuration, the permeated fluid can be discharged and collected through the discharge port provided at the casing""s side while the feed fluid that has passed through the open channel can be circulated through a circulatory line connected via a connector, and thus repeatedly fed to the filter.
In the present invention, preferably the casing is formed of at least one material selected from the group consisting of plastic, metal and ceramic. With such configuration, there can be provided a filter which is highly resistant to pressure, heat and chemicals.
In the present invention, preferably the channel has opposite ends sealed with a seal formed of at least one material selected from the group consisting of plastic, metal and ceramic. With such configuration, there can be provided a filter which is highly resistant to pressure, heat and chemicals.
In the present invention, preferably the casing has a cross section having an outer diameter from 10 mm to 500 mm and a length of 100 mm to 2000 mm passing a fluid. With such a configuration, normally the casing can readily be transported and mounted and for the casing with such cross sectional dimension a filter unit used in conjunction with the casing can be readily manufactured.
In the present invention, preferably the channel is arranged at a pitch of 0.5 mm to 3 mm. With such configuration, required resistance for stress and separation capability can be obtained while superior permeability can be maintained.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.